This invention relates to a loudspeaker telephone device or set and to a noise suppressing circuit which is for use typically in the loudspeaker telephone device.
Usually, a telephone set includes a handset comprising a transmitter and a receiver. A microphone is generally used as the transmitter. When the telephone set is used in a mobile telephone substation installed in an automobile or a like vehicle, where it is highly desired for a user to talk to and to receive a talk signal from a person at a counterpart telephone substation of a plurality of remote telephone substations without holding the handset, a loudspeaker is preferably used instead of the receiver housed in the handset. Such a telephone set is herein called a loudspeaker telephone device.
A loudspeaker telephone device of a local telephone substation converts a microphone output signal to a talk transmission signal, which is transmitted to a counterpart telephone substation typically as a radio transmission signal. When received as a radio reception signal from the counterpart telephone substation, a talk reception signal is converted by the loudspeaker telephone device to a loudspeaker drive signal for use in driving the loudspeaker.
It goes without saying that a mobile telephone substation is for transmitting a user's talk to a counterpart telephone substation and for receiving signal a talk sent from the counterpart telephone substation in order to listen. On transmitting the user's talk to the counterpart telephone substation, the loudspeaker telephone device should be put in a talk transmission mode of operation of giving a small amount of attenuation to the microphone output signal and of giving a great attenuation to the talk reception signal which is quiescent in practice. On receiving a talk signal from the counterpart telephone substation, the loudspeaker telephone device should be put in a talk reception mode of operation of giving a small amount of attenuation to the talk reception signal and a great amount of attenuation to the microphone output signal which may have an appreciably high level due to noise surrounding the loudspeaker telephone device.
In the manner which will later be described more in detail, a conventional loudspeaker telephone device comprises a comparator unit for comparing the microphone output signal having a microphone signal level with the loudspeaker drive signal having a loudspeaker drive level. The comparator unit thereby produces an attenuation control signal with an attenuation control level which is inversely proportional to the microphone signal level less the drive level, namely, inversely proportional to a level difference between the microphone signal level and the drive level. In the manner described in the following, the attenuation control signal is used in putting the loudspeaker telephone device automatically selectively in the talk transmission and reception modes.
In the loudspeaker telephone device, a first attenuator unit is supplied with the microphone output signal and controlled by the attenuation control signal to produce the talk transmission signal by giving the microphone output signal a first variable amount of attenuation which increases with an increase in the attenuation control level. A second attenuator unit is supplied with the talk reception signal and controlled by the attenuation control signal to produce the loudspeaker drive signal by giving the talk reception signal a second variable amount of attenuation which decreases with the above-mentioned increase.
The microphone of the mobile telephone substation is often surrounded by a strong or loud noise. In such an event, the microphone produces the microphone output signal with a considerably high microphone signal level even when the user does not speak to the microphone in the meantime. As a consequence, the microphone signal level becomes appreciably higher than the drive level even if the talk reception signal is delivered to the second attenuator unit with a significant level. Under the circumstances, the conventional loudspeaker telephone device is undesiredly kept in the talk transmission mode of attenuating the talk reception signal. This makes the loudspeaker drive signal have an objectionably low drive level and renders it impossible for the loudspeaker to clearly reproduce the talk sent from the counterpart telephone substation.